There are usually two "issues" with relationship which can be solved by the "let's meet up in x years" thing.
First is distance (which however assumes you will work to overcome it) and second is having some previous attachments (e.g. signing up for army, etc.) which would ruin the relationship but will not ruin friendship and possible chance of relationship after it ends (but it also assumes that neither of you will look for a partner in the meantime).
This is - from what I gather - neither.
So what is the gain?
For him, it's easy to pinpoint: he has a safety belt against loneliness. In case all his future relationships fail, you'll be there for him (not as a friend, but as... supplement). And he doesn't have to be afraid of failing in relationships.
You know how people get more attractive when they are not single? Because they are not desperate to find a mate - so they are more at ease. By "staying by his side", waiting for the future relationship, you are giving him the advantage of being more attractive by not having to find a mate. After all, if it doesn't work he only has to call you.
For you... not so much. He's the one with certainty - and you're his certainty.
You know one sure way to ruin any relationship? Take someone for granted. I see relationships that burn due to this all the time - you take someone for granted, soon they become "non-special". Along comes someone special - not better, just someone who is not a certainty. The End.
But one thing - we can talk, you can read it - but the result is, unless you decide, on your own, that you want a working relationship and are willing to do something for it... yeah, it's a losing battle. It will only lead to you posting there, people offering sympathy/advice and you going deeper and deeper into the personal hell of seeing someone you want to spend your life with being with other people. And being happy.
Do you want to be happy? Do you want to have a working relationship?
Prepare to work for it.
Rant end
...and by the way... it can be done. You are able - and you have right for a healthy, working relationship, in which you are happy. Don't throw it away.